05000336/LER-2013-003
| Millstone Power Station - Unit 2 | |
| Event date: | 10-30-2013 |
|---|---|
| Report date: | 12-18-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 49487 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| LER closed by | |
| IR 05000245/2016008 (4 January 2017) | |
| 3362013003R00 - NRC Website | |
1. EVENT DESCRIPTION
While Millstone Power Station Unit 2 (MPS2) was operating at 100 percent power in Mode 1 on October 30, 2013, operations determined that a concern identified by Engineering as a result of recent industry operating experience could result in an unanalyzed condition. Engineering's review of the impact of unfused direct current (DC) ammeter [II] circuits in the Control Room determined the described condition to be applicable to MPS2. It is postulated that a fire could cause one of the DC ammeter wires to hot short to ground. Concurrently, the fire causes another DC wire from the opposite polarity on the same battery to also short to ground. This would cause a current path through the unfused ammeter cable. The original plant wiring design and associated circuitry analysis for the batteries' control room ampere indications do not include overcurrent protection features to limit the fault current in this scenario.
The following MPS2 components have the vulnerability identified, i.e., DC current measuring circuits connected to a 125VDC power [El] source without a protective device.
- Inverters #1 through.#6
- DC1 "A" Battery Charger [BYC] to Battery Bus [BU] 201A
- DC2 "B" Battery Charger to Battery Bus 201B
- DC4 125 VOLT DC Turbine [MT] Battery Charger
- 201A DC Battery Bus 201A (D01)
- 201B DC Battery Bus 201B (D02)
- 201D Turbine Battery Bus (D03) Fire areas affected because of the postulated hot shorts in DC circuits with ammeters are the battery rooms, the DC switchgear rooms, the turbine battery room located in the 31' 6" elevation of the turbine building, the 25' elevation cable vault, the old and new computer rooms, the plant equipment operator (PEO) meeting area, the west 480V switchgear room, and the control room.
This condition is reportable pursuant to 10 CFR 50.73(a)(2)(ii)(B) as an unanalyzed condition that significantly degraded plant safety. A prompt notification per EN49487 was submitted to the NRC on October 30, 2013.
2. CAUSE
The cause of the condition was a latent design error that was made during plant construction.
3. ASSESSMENT OF SAFETY CONSEQUENCES
Fire areas affected because of the postulated hot shorts in DC circuits with ammeters in the equipment listed above are the battery rooms, the DC switchgear rooms, the turbine battery room located in the 31' 6" elevation of the turbine building, the 25' elevation cable vault, the old and new computer rooms, the plant equipment operator (PEO) meeting area, the west 480V switchgear room, and the control room.
The zones where the initial fire occurs that could lead to a secondary fire are the west 480V and DC switchgear rooms, the 31'6" elevation of the turbine building, and the PEO meeting area.
A reportable condition' would only occur if a secondary fire occurs which affects the opposite train equipment. This could only occur if the secondary fire occurs in the control room or the 25' cable vault.
In order for this low probability occurrence to result in a secondary fire, the opposite pole short must develop, and it must not be mitigated by other circuitry protection schemes, such as fuses present in the affected cables. Furthermore, if multiple faults were to occur, they would be limited in number, small in magnitude, and short in duration.
The majority of these fire areas have fire detection installed and several have reliable automatic fire suppression systems. The control room is constantly manned by personnel who are trained to extinguish fires. Therefore, any low energy secondary fire will be quickly detected and extinguished. On this basis, the safety consequences of these postulated secondary fires is considered low.
4. CORRECTIVE ACTION
Compensatory measures, i.e., fire watches, have been implemented for affected areas of the plant. Plant modifications are planned that will protect or isolate the affected circuits leaving the initial fire area.
Additional corrective actions are being taken in accordance with the Station's corrective action program.
5. PREVIOUS OCCURRENCES
No previous similar occurrences.
Energy Industry Identification System (EllS) codes are identified in the text as [OX].